To be specific, we've used Charmin Ultra Soft in our RV's for years without any problems. It does pass the "water test", and even though it takes a bit longer than some others to break down, it's still faster than I can pull up my britches and run outside to dump the tank, so no worries...

A simple TP check is to take a piece of your preferred brand of toilet paper and place it in a glass full of water and shake it up to see if it dissingrates. If it does, it will be safe to use. If it stays intact, then it may cause black tank issues.